Conductive patterns on polymeric substrates such as, e.g., printed circuit boards typically are used to provide interconnections between devices attached to such substrates. Several methods have been used for the production of these conductive patterns; two of these, dry-film imaging and screen printing, are briefly described below.
Dry-film imaging typically involves the following steps: Electroless copper is deposited on a substrate. (The substrate may have holes for through-hole connection, in which case an acid copper flash is then applied to the substrate to add extra copper through the holes.) The copper-plated substrate is baked, a dry-film photoresist is applied to the copper coating, and the photoresist is exposed to actinic light patterned by artwork. The exposed photoresist is developed, unexposed resist is removed, and exposed copper is etched away.
Screen-printing utilizes a photomechanical stencil attached to a precision-woven fabric stretched over a frame. The stencil is attached to the fabric by soaking the fabric in a light-sensitive emulsion and exposing selected areas to actinic light; areas exposed to light become insoluble in water. The fabric is then rinsed leaving areas of open mesh. The resist is forced through the stencil onto a plated substrate, resist passing only through open mesh areas. The substrate is then exposed to an etchant which etches away those portions of the plating layer which were left uncovered by the resist.
For further details on these processes see, e.g., the book by R. H. Clark, Handbook of Printed Circuit Manufacturing, Van Nostrand Reinhold Co., 1985.